Supersoft Skin

H²O equals hydration, right? Not necessarily. When moisture evaporates from skin, it can take naturally hydrating lipids lingering below the surface with it. And the kind of water you're dipping into when the weather heats up wreaks even more havoc: "Chlorine is extremely drying, and salt is hydrophilic—it's attracted to water—and pulls water from skin," says Adam Geyer, M.D., a dermatologist in New York City. Keep skin supple with one easy step.

Suffuse skin with moisture. Before you put on that first layer of sunscreen, apply a skin-quenching oil. "Oils form a light coating that seals in moisture," says Sapna Westley, M.D., a dermatologist in NYC. "But even more important, oils are made up of tinier molecules than creams, so they can penetrate skin more deeply, filling up the tiny spaces between cells, where natural lipids lie."

Make-It-Last Makeup

To stop smudges, swap your usual formulas for more tenacious versions containing aqua-blocking silicones (such as dimethicone) and polymers (labels might list acrylate or octylacrylamide), says James Hammer, a cosmetic chemist in Easton, Massachusetts. Then amplify their lasting power with these simple stay-pretty tricks.

Avoid a mascara meltdown. Waterproof mascaras contain nifty polymers that latch on to lashes, but they might not hold up if you're in the water all day. To enhance endurance, dust translucent powder over bare lashes first. "Powder absorbs any trace of oil on lashes, so mascara adheres even better," says Tina Turnbow, a makeup artist in NYC.

Shield skin from sweat. When the temperature soars, sweat glands go into overdrive to cool you down. That's a good thing, but the possible oil slick that ensues? Not so much. Instead of gobbing on powder to sop up shine, lightly pat a pore-minimizing cream over foundation. It will help absorb excess oil but won't rob skin of radiance.

Healthy, Shiny Hair

Nothing unravels a style (even one that's meant to look beachy and tousled) faster than sweat or a blast of humid air. And just as with every spin cycle your favorite jeans fade, water—whether tap, chlorinated or salt—can take haircolor from vibrant to washed out. Maintain your color and texture with these style savers.

Protect your color. Even a quick swim can send hair (especially highlights and allover blonde) into Shrek territory. "Water contains copper, which binds to proteins inside hair," says Sabrina Michals, lead styling educator for Bumble and Bumble salon in NYC. When chlorine oxidizes that copper, your hair can turn green. (Witness the Statue of Liberty—once brown, now sea-green.) Remove buildup and brighten your color, whatever the shade, with a gentle clarifying treatment: Wash with 1/2 cup of shampoo to which you've added 1 tablespoon of baking soda, then reshampoo with a regular formula and conditioner.

Take control of texture. Heat, humidity and water create the perfect storm for frizz. To keep wiry strands on lockdown, use an oil-based leave-in conditioner, suggests Lorri Goddard-Clark, a haircolorist in Los Angeles. It's basic chemistry: Oil and water don't mix.